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Re: Note on DTD-related questions

From: Murray Altheim <altheim@eng.sun.com>
Date: Wed, 14 Jun 2000 16:26:03 -0700
Message-ID: <3948148B.7543C056@eng.sun.com>
CC: w3c-xml-schema-ig@w3.org, www-xml-schema-comments@w3.org
I'm sorry that I haven't been able to provide a detailed summary of
comments on the hard work that Michael, Eve and Norm provided in 
their response to Ann Navarro and my comments on the XML Schema 
specification [1].

While my present schedule has prevented this summary from being made
public, I think that given the current deadline for comments that 
I'd at least like it to be known that I don't believe the solutions 
provided by [1] to be sufficient to enable definition of XHTML or 
particularly XHTML's modularization using the XML Schema grammar; that
while it is demonstrably possible to enable certain constructs (and
to certainly allow for those *not* in DTDs), that the manner in which
DTDs and XML Schemas constrain structures, and the flexibilities 
inherent in parameter entities and conditional sections do not find
a sufficient mirror in XML Schemas.

For example, the ability to easily redefine a content model is not
a feature available in XML Schemas, except for the less-than-
interesting case of appending element types at its end. Ie., it's 
impossible to add a <preface> near the beginning of a content model
if that is desired without modifying the base Schema itself. In a 
DTD this is accomplished rather easily by redeclaring the content 
model in almost any number of places. This is a common requirement 
that is not satisfied (as far as I can understand from the 
Structures spec and the "Note on DTD-related questions". And any
possible solution shouldn't require any rocket-science: the solution
in modular DTDs is fairly straightforward if the DTD is regular
and well-documented.
Without going into great detail I think the consequences of the 
rather profound differences between the flexibilities inherent in 
highly parameterized DTDs (given both the pros and cons therein) 
and the well-structured and inheritance-based constraints in XML 
Schemas mean that certain features of DTDs are either impossible
or impractical in Schemas, and vice-versa. I do not find XML Schemas
to be amenable to XHTML modularization *if* that is a priority. I
might note that the idea of striving toward the creation of a data-
centric schema for a document-centric vocabulary might in the end
be less than necessary or important, ie., the project itself might
be a lesson in futility, less than valuable for any particular 
community, a marketing exercise perhaps.

Again, I apologize that I have not been able to be more explicit
in my response, and would endeavour to clarify any comments that
I have made if there seems to be any misunderstanding. I hope that
this note and any subsequent comments fulfills my responsibility 
in commenting on these specifications. 

Thank you all for your patience,


[1] "Note on DTD-related questions", Eve Maler, C. M. Sperberg-McQueen,
Norm Walsh, 20 April 2000
Murray Altheim                            <mailto:altheim&#x40;eng.sun.com>
XML Technology Center
Sun Microsystems, Inc., MS MPK17-102, 1601 Willow Rd., Menlo Park, CA 94025

      In the evening
      The rice leaves in the garden
      Rustle in the autumn wind
      That blows through my reed hut.  -- Minamoto no Tsunenobu
Received on Wednesday, 14 June 2000 19:26:35 UTC

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